1. Field of the Invention
The invention relates to a ceramic wiring board having a non-oxide based ceramic substrate which has aluminum nitride or silicon nitride as main component and a method of producing the same.
2. Description of the Related Art
Demands for a ceramic substrate are increasing year by year with high integration, speeding-up, high power consuming and large integration of semiconductor devices.
Especially, since the aluminum nitride (AlN) substrate has a high thermal conductivity, outstanding thermal radiation and also a linear thermal expansion coefficient which is close to that of Si, it is expected as circuit board and semiconductor package substrate instead of an alumina (Al.sub.2 O.sub.3) substrate.
A silicon nitride (Si.sub.3 N.sub.4) substrate is poor in thermal conductivity as compared with the aluminum nitride substrate but has a linear thermal expansion coefficient close to that of Si and also excels in a mechanical strength and a fracture toughness value. And, its adaption for the circuit board and semiconductor package substrates is now being made. Besides, the silicon nitride substrate can be given a thermal conductivity of 40 W/mK or more which is higher than that of the alumina substrate by controlling the particle size of the silicon nitride powder and the sintering auxiliary composition, and its practical utility is being acknowledged as semiconductor substrate material.
To use the above-described aluminum nitride and silicon nitride substrates as circuit board or package substrate on which electronic parts are mounted, a metallized layer formed of W, Mo, Ni, Cr, Pd, Ag, Al, Mn or the like is formed as electrode, wiring layer or the like.
This metallized layer is formed by simultaneous sintering with the substrate or by applying and baking a conductive paste after sintering the substrate.
And, a metal plated layer formed of Ni, Au, Cu, Ag or a combination thereof is generally formed on the metallized layer in order to provide a joining property with a solder or a wire bonding property.
A liquid-phase compound, which is formed of one member or a compound of two or more members of Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, AlN, WN, WO.sub.3, SiO.sub.2 or SiN while the substrate is being sintered, is present on the surface of the metallized layer, especially the surface of the metallized layer formed by the simultaneous sintering with the substrate. Such a compound is produced because a liquid-phase component which mainly has the compound added as sintering auxiliary reacts with carbon contained in a sintering atmosphere while aluminum nitride or silicon nitride is densified, and moves outside (expelled) of the aluminum nitride substrate or silicon nitride substrate. Therefore, in addition to the liquid-phase compound described above, a compound of carbon with the component elements of the liquid-phase compound is also present on the surface of the metallized layer.
The layer comprising the above-described liquid-phase compound and the compound of carbon with the liquid-phase compound interrupt the metal plated layer from being deposited on the metallized layer.
The metallized layer is generally wet-etched with an acid solution or alkaline solution prior to forming the metal plated layer on the metallized layer. To perform the wet etching, an alkaline solution of NH.sub.4 OH, NaOH, KOH, potassium ferricyanide, KCN or the like is generally used, but such an alkaline solution has a disadvantage that it cannot selectively etch the interrupting layer only because it acts as etchant on the aluminum nitride substrate, the silicon nitride substrate and other non-oxide based ceramic substrates.
Therefore, selection of etching conditions for thorough removal of the interrupting layer results in deterioration or changing the properties of the aluminum nitride substrate, the silicon nitride substrate or the like.
If the selected etching conditions can prevent the aluminum nitride substrate, the silicon nitride substrate or the like from being deteriorated or having its properties changed, the interrupting layer cannot be removed thoroughly, so that the metal plated layer cannot be deposited satisfactorily. In other words, the metal plated layer cannot be formed to a sufficient thickness, its thickness becomes uneven, and its adhesion strength is degraded.
Especially, recent semiconductor packages have the electrode, particularly an area of the metallized layer, made small in order to achieve atomizing or high densifying of an I/O pattern, so that an adverse effect by the interrupting layer is increased.
As described above, no technology has been achieved to remove the interrupting layer comprising the liquid-phase compound or the compound of carbon with the liquid-phase compound from the metallized layer formed on the non-oxide based ceramic substrate such as the aluminum nitride substrate or the silicon nitride substrate. And, the metal plated layer cannot be deposited satisfactorily on the metallized layer.
In view of above, it is demanded to secure a thickness of the metal plated layer, a uniform thickness, and improvement of an adhesion strength with respect to the non-oxide based ceramic substrate having the metallized layer.
And, the wet etching has a disadvantage that when the non-oxide based ceramic substrate or the metallized layer is porous, an etching solution is easy to soak therein, and the solution exudes to the surface after plating to form a reaction product, thereby easily causing discoloration. This disadvantage is also demanded to be remedied.